An array of acoustic transducers can be used to direct and focus acoustic energy into a small target in order to heat the target. To accomplish the bending and focussing, prisms and lenses must be used in front of each transducer. Each prism/lens must be precisely positioned relative to the transducers and relative to each other in order to achieve the desired focussing effect. The cumulative weight of the lenses and the framing needed for positioning can be a problem when it is necessary to change the lens array. Thick lenses cause problems of absorption. Highly curved lenses cause problems of scattering.